Lately, x-radiation has been extensively used in the security field for inspecting objects in airports, courthouses, federal buildings, etc. Various systems have been proposed to insure adequate resolution of an image at a relatively low intensity of radiation. For example, U.S. Pat. No. RE 28,544 to Stein et al describes an X-ray source generating radiation which is collimated to form a flying pencil beam for scanning a line in space. An object to be scanned is positioned in front of the X-ray source. An X-ray detector is then positioned farther away from the X-ray source than the object and directly behind the object at the line in space. After the X-ray source is turned on, the incident photons are converted by the detector into electrical signals which drive a video display. If the object to be inspected is translated in a direction transverse to the scanning line, the object can be fully scanned in the X-Y plane. This so-called forward transmitted detector can only detect those X-rays which are transmitted through the object without substantially changing their direction. Some X-rays do not reach the detector because they are absorbed by a component in the object. This results in a shadow which appears on the video display as an outline of that X-ray absorbing component.
The described use of X-ray illumination and imaging has proved particularly effective in detecting relatively dense, solid materials, such as conventional metal weapons, etc. Having a high atomic number (high Z), these dense materials absorb X-rays and produce an easily identifiable image displaying a shape of an illegal article. A security officer can then readily recognize certain distinct shapes, i.e., a handgun, etc., based on the video image.
Complications with detection, however, arise with materials having a low atomic number (low Z), such as plastic weapons, drugs, etc. These objects do not absorb but scatter X-rays producing fuzzy, vague and practically unrecognizable images of components made of low Z materials. Having this attribute, plastic or ceramic illegal objects are quite difficult to detect during inspections even by trained professionals.
Since low Z materials fail to produce clear, easily recognizable images using conventional techniques of the forward transmitted detector which only captures penetrating x-rays, various methods and systems have been proposed to solve the problem of detecting these materials using X-ray scanning and imaging. U.S. Pat. Nos. 4,799,247 and 5,313,511, both to Armis et al, and U.S. Pat. No. 5,181,234 to Smith illustrate systems for detecting, inter alia, backscatter radiation from low Z materials. These patents disclose systems for illuminating low Z materials and capturing the backscattered or reflected radiation onto a photosensitive detector. In contrast to the conventional technique of placing the detector behind an object to be illuminated by an X-ray source, the backscatter detector is positioned on the same side of the illuminated object as the X-ray source. The backscatter detector then provides an electrical signal representative of the intensity of the X-rays scattered from the object being scanned. Based on the electrical signal, an image of the object is shown on a video display for visual detection and identification of illegal materials.
An important property of any X-ray image is contrast. For example, when one low Z object appears in an image on top of another low Z object, such as the human body, the contrast between these two objects can be low. Similarly, a high Z object viewed against the background of another high Z object will also display low contrast. Consequently, the objects in the X-ray image appear as indistinct, without well defined outlines or edges.
Therefore, to significantly improve inspection of objects, either animate or inanimate, as disclosed by the prior art, the present invention is provided for enhanced edge detection and enhancement of materials using X-ray illumination and imaging.